This invention relates to a device for providing level indication of solid particles flowing in a vessel, bin or pipe; most particular for providing level indication of flowing wood chips in a continuous wood chip digester.
Known methods for determining the level of moving solid particles in a vessel usually involve a device that indicates positively whether the solid is or is not at the location of the indicator. The positive indication is normally provided by devices that fall in the following categories: Nuclear source and detection of rays across the vessel; sonic source with detection of the sound wave across the vessel; or magnetic field producing devices that detect the presence of solid by changes in the field. All of the devices for level detection in the above categories function adequately for solids level indication except for one common fault; they lack the ability to adequately sense solids levels in vessels filled with liquids. When the vessel is filled with a liquid the liquid itself as well as small suspended solids distort the rays, sonic waves and magnetic fields causing uncertainty in the level indication. The present invention uniquely contains the ability to indicate a solid particle level whether the solid particles are moving downward in a liquid filled vessel or a dry vessel.
According to the present invention a device is provided for indicating positively the presence of a flowing solid. The device specifically consists of a flat plate or blade located transverse to the motion or flow direction of the solid particles. One end of the plate is attached rigidly and perpendicular to a shaft. The shaft may be attached rigidly to the vessel wall by a flanging and sealing means. Strain gauges are preferably located in a hollow portion of the shaft near the flange connection, the hollow portion not being at the vessel pressure. Also the shaft material thickness is smallest at the hollow area. The motion of solid particles transverse to the plate, apply a force to the plate which results in a twist on the shaft. The twist is reflected as a physical strain at the hollowed out area in the shaft, said strain being monitored by one or multiple strain gauges adhered to the inside surface of the shaft. When so solids are flowing at the plate location the strain will be very small, thus indicating that the solid particle level is below the plate.
The level indicator of the present invention may include a flange mounting mechanism which contains pressure seals, means for removal from the vessel and shear pins to allow the plate to rotate to in line with the solid particle flow should the force on the plate become large enough to twist the shaft to breakage. The shearing of pins is most effective when solid particles become hung up above the indicator and then drop as a mass, applying a very large force on the plate. In such a case, the shear pins will shear to allow the shaft to rotate to prevent shaft breakage. The shear pins are conveniently located outside the pressure seals to allow pin replacement after the shaft and plate are rotated to the operating position.
It is a primary object of the present invention to provide an improved solids level detection device, especially for a cellulose digester. This and other objects of the invention will become clear from an inspection of the detailed description of the drawings and the appended claims.